Embedded SONOS based memory cells

ABSTRACT

Memory cells including embedded SONOS based non-volatile memory (NVM) and MOS transistors and methods of forming the same are described. Generally, the method includes: forming a dielectric stack on a substrate, the dielectric stack including a tunneling dielectric on the substrate and a charge-trapping layer on the tunneling dielectric; patterning the dielectric stack to form a gate stack of a NVM transistor of a memory device in a first region of the substrate while concurrently removing the dielectric stack from a second region of the substrate; and performing a gate oxidation process of a baseline CMOS process flow to thermally grow a gate oxide of a MOS transistor overlying the substrate in the second region while concurrently growing a blocking oxide overlying the charge-trapping layer. In one embodiment, Indium is implanted to form a channel of the NVM transistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. 119(e)to U.S. Provisional Patent Application Ser. No. 61/769,693, filed Feb.26, 2013, and to U.S. Provisional Patent Application Ser. No.61/825,196, filed May 20, 2013, both of which are incorporated byreference herein.

TECHNICAL FIELD

The present disclosure relates generally to semiconductor devices, andmore particularly to memory cells including embedded or integrallyformed SONOS based non-volatile memory (NVM) transistors andmetal-oxide-semiconductor (MOS) transistors and methods for fabricatingthe same.

BACKGROUND

For many applications, such as system-on-chip, it is desirable tointegrate logic devices and interface circuits based uponmetal-oxide-semiconductor (MOS) field-effect transistors andnon-volatile memory (NVM) transistors on a single chip or substrate.This integration can seriously impact both the MOS transistor and NVMtransistor fabrication processes. MOS transistors are typicallyfabricated using a standard or baselinecomplimentary-metal-oxide-semiconductor (CMOS) process flows, involvingthe formation and patterning of conducting, semiconducting anddielectric materials. The composition of these materials, as well as thecomposition and concentration of processing reagents, and temperatureused in such a CMOS process flow are stringently controlled for eachoperation to ensure the resultant MOS transistors will functionproperly.

Non-volatile memory devices include non-volatile memory transistors,silicon-oxide-nitride-oxide-semiconductor (SONOS) based transistors,including charge-trapping gate stacks in which a stored or trappedcharge changes a threshold voltage of the non-volatile memory transistorto store information as a logic 1 or 0. Charge-trapping gate stackformation involves the formation of a nitride or oxynitridecharge-trapping layer sandwiched between two dielectric or oxide layerstypically fabricated using materials and processes that differsignificantly from those of the baseline CMOS process flow, and whichcan detrimentally impact or be impacted by the fabrication of the MOStransistors. In particular, forming a gate oxide or dielectric of a MOStransistor can significantly degrade performance of a previously formedcharge-trapping gate stack by altering a thickness or composition of thecharge-trapping layer. In addition, this integration can seriouslyimpact the baseline CMOS process flow, and generally requires asubstantial number of mask sets and process steps, which add to theexpense of fabricating the devices and can reduce yield of workingdevices.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood more fully from the detaileddescription that follows and from the accompanying drawings and theappended claims provided below, where:

FIG. 1 is a flowchart illustrating an embodiment of a method forfabricating a memory cell including an embeddedsilicon-oxide-nitride-oxide-semiconductor (SONOS) based non-volatilememory (NVM) transistor and metal-oxide-semiconductor (MOS) transistors;

FIGS. 2A-2M are block diagrams illustrating cross-sectional views of aportion of a memory cell during fabrication of the memory cell accordingto the method of FIG. 1;

FIG. 2N is a block diagram illustrating a cross-sectional view of aportion of a finished memory cell including an embedded SONOS based NVMtransistor and MOS transistors fabricated according to the method ofFIGS. 1 and 2A-2M;

FIGS. 3A and 3B are graphs illustrating improvements in thresholdvoltage (V_(T)) uniformity of an NVM transistor including an indiumchannel according to an embodiment of the present disclosure; and

FIGS. 4A-4C are graphs illustrating basic program erase characteristicsand showing improvements in endurance data retention for a memory deviceincluding an embedded SONOS based NVM transistor fabricated according toan embodiment of the present disclosure.

DETAILED DESCRIPTION

Embodiments of a memory cell including an embedded non-volatile memory(NVM) transistor and a metal-oxide-semiconductor (MOS) transistor andmethods of fabricating the same are described herein with reference tofigures. However, particular embodiments may be practiced without one ormore of these specific details, or in combination with other knownmethods, materials, and apparatuses. In the following description,numerous specific details are set forth, such as specific materials,dimensions and processes parameters etc. to provide a thoroughunderstanding of the present invention. In other instances, well-knownsemiconductor design and fabrication techniques have not been describedin particular detail to avoid unnecessarily obscuring the presentinvention. Reference throughout this specification to “an embodiment”means that a particular feature, structure, material, or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the invention. Thus, the appearances of the phrase “in anembodiment” in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, materials, orcharacteristics may be combined in any suitable manner in one or moreembodiments.

The terms “over,” “under,” “between,” and “on” as used herein refer to arelative position of one layer with respect to other layers. As such,for example, one layer deposited or disposed over or under another layermay be directly in contact with the other layer or may have one or moreintervening layers. Moreover, one layer deposited or disposed betweenlayers may be directly in contact with the layers or may have one ormore intervening layers. In contrast, a first layer “on” a second layeris in contact with that second layer. Additionally, the relativeposition of one layer with respect to other layers is provided assumingoperations deposit, modify and remove films relative to a startingsubstrate without consideration of the absolute orientation of thesubstrate.

The NVM transistor may include memory transistors or devices implementedusing Silicon-Oxide-Nitride-Oxide-Silicon (SONOS) or floating gatetechnology.

An embodiment of a method for integrating or embedding a NVM transistorinto a standard or baseline CMOS process flow for fabricating one ormore MOS transistors will now be described in detail with reference toFIG. 1 and FIGS. 2A through 2M. FIG. 1 is a flowchart illustrating anembodiment of a method or process flow for fabricating the memory cell.FIGS. 2A-2L are block diagrams illustrating cross-sectional views of aportion of a memory cell during fabrication of the memory cell accordingto the method of FIG. 1, and FIG. 2M is a block diagram illustrating across-sectional view of a portion of an embodiment of the finishedmemory cell.

Referring to FIG. 1 and FIG. 2A, the process begins with forming anumber of isolation structures 202 in a wafer or substrate 204 (step102). The isolation structures 202 isolate the memory cell being formedfrom memory cells formed in adjoining areas (not shown) of the substrate204 and/or isolate the NVM transistor being formed in a first region 206of the substrate from one or more of the MOS transistors transistorbeing formed in a second region 208. The isolation structures 202include a dielectric material, such as an oxide or nitride, and may beformed by any conventional technique, including but not limited toshallow trench isolation (STI) or local oxidation of silicon (LOCOS).The substrate 204 may be a bulk wafer composed of any single crystalmaterial suitable for semiconductor device fabrication, or may include atop epitaxial layer of a suitable material formed on a substrate.Suitable materials include, but are not limited to, silicon, germanium,silicon-germanium or a III-V compound semiconductor material.

Generally, as in the embodiment shown, a pad oxide 209 is formed over asurface 216 of the substrate 204 in both the first region 206 and thesecond region 208. The pad oxide 209 can be silicon dioxide (SiO₂)having a thickness of from about 10 nanometers (nm) to about 20 nm andcan be grown by a thermal oxidation process or in-situ steam generation(ISSG).

Referring to FIG. 1 and FIG. 2B, dopants are then implanted intosubstrate 204 through the pad oxide 209 to form wells in which the NVMtransistor and/or the MOS transistors will be formed, and channels forthe MOS transistors (step 104). The dopants implanted may be of any typeand concentration, and may be implanted at any energy, includingenergies necessary to form wells or deep wells for the NVM transistorand/or the MOS transistors, and to form channels for the MOStransistors. In a particular embodiment illustrated in FIG. 2B, dopantsof an appropriate ion species are implanted to form a deep N-well 210 inthe second region 208 over or in which a high-voltage (HV) MOStransistor 214, such as a MOS input/output (I/O) transistor, will beformed. Although not shown, it is to be appreciated that the wells ordeep wells can also be formed for the NVM transistor and/or a standardor low-voltage (LV) MOS transistor, such as a MOS transistor 212. It isfurther to be appreciated that the wells are formed by depositing andpatterning a mask layer, such as a photoresist layer above the surface216 of the substrate 204, and implanting an appropriate ion species atan appropriate energy to an appropriate concentration.

Channels 218 for one or more of the MOS transistors 214, 212, are formedin the second region 208 of the substrate 204. As with the well implantthe channels 218 are formed by depositing and patterning a mask layer,such as a photoresist layer above the surface 216 of the substrate 204,and implanting an appropriate ion species at an appropriate energy to anappropriate concentration. For example, BF₂ can be implanted at anenergy of from about 10 to about 100 kilo-electron volts (keV), and adose of from about 1e12 cm⁻² to about 1e14 cm⁻² to form an N-type MOS(NMOS) transistor. A P-type MOS (PMOS) transistor may likewise be formedby implantation of Arsenic or Phosphorous ions at any suitable dose andenergy. It is to be appreciated that implantation can be used to formchannels 218, in both of the MOS transistors 214, 212, at the same time,or at separate times using standard lithographic techniques, including apatterned photoresist layer to mask one of the channels for the MOStransistors.

Next, referring to FIG. 1 and FIG. 2C a patterned tunnel mask 220 isformed on or overlying the pad oxide 209, ions (represented by arrows222) of an appropriate, energy and concentration are implanted through awindow or opening in the tunnel mask to form a channel 224 for a NVMtransistor 226, and the tunnel mask and the pad oxide in at least thesecond region 208 removed (step 106). The tunnel mask can include aphotoresist layer, or a hard mask formed, from a patterned nitride orsilicon-nitride layer.

In one embodiment, the channel 224 for the NVM transistor 226 is anIndium doped channel implanted with Indium (In) at an energy of fromabout 50 to about 500 kilo-electron volts (keV), and a dose of fromabout 5e11 m⁻² to about 5e12 cm⁻² to form a n-channel NVM transistor. Asexplained in greater detail below, implanting Indium to form the channel224 of the NVM transistor 226 improves the threshold voltage (V_(T))uniformity of the NVM transistor from a sigma of V_(T) from about 150millivolts (mV) to from about 70 to 80 mV. Alternatively, BF₂ can beimplanted to form an n-channel NVM transistor, or Arsenic or Phosphorousimplanted to form a p-channel NVM transistor.

A photoresist tunnel mask 220 can be ashed or stripped using oxygenplasma. A hard mask can be removed using a wet or dry etch process. Thepad oxide 209 is removed, for example in a wet clean process using a10:1 buffered oxide etch (BOE) containing a surfactant. Alternatively,the wet clean process can be performed using a 20:1 BOE wet etch, a 50:1hydrofluoric (HF) wet etch, a pad etch, or any other similarhydrofluoric-based wet etching chemistry.

Referring to FIG. 1 and FIGS. 2D-2F, the surface 204 of the substrate204 is cleaned or precleaned, a number of dielectric layers, such asoxide-nitride-oxide or ONO layers, formed or deposited, a mask formed onor overlying the dielectric layers, and the dielectric layers etched toform a dielectric gate stack 236 in the first region 206 (step 108). Thepreclean can be a wet or dry process and in this embodiment is wetprocess using HF or standard cleans (SC1) and SC2, and is highlyselective to the material of the substrate 204. SC1 is typicallyperformed using a 1:1:5 solution of ammonium hydroxide (NH₄OH), hydrogenperoxide (H₂O₂) and water (H₂O) at 50 to 80° C. for about 10 minutes.SC2 is a short immersion in a 1:1:10 solution of HCl, H2O2 and H₂O atabout 50 to 80° C.

Referring to FIG. 2D, the dielectric or ONO deposition begins with theformation of a tunnel dielectric 228 over at least the channel 224 ofthe NVM transistor 226 in the first region 206 of the substrate 204. Thetunnel dielectric 228 may be any material and have any thicknesssuitable to allow charge carriers to tunnel into an overlyingcharge-trapping layer under an applied gate bias while maintaining asuitable barrier to leakage when the NVM transistor is unbiased. Incertain embodiments, tunnel dielectric 228 is silicon dioxide, siliconoxy-nitride, or a combination thereof and can be grown by a thermaloxidation process, using ISSG or radical oxidation.

In one embodiment a silicon dioxide tunnel dielectric 228 may bethermally grown in a thermal oxidation process. For example, a layer ofsilicon dioxide may be grown utilizing dry oxidation at 750 degreescentigrade (° C.)-800° C. in an oxygen containing gas or atmosphere,such as oxygen (O₂) gas. The thermal oxidation process is carried outfor a duration approximately in the range of 50 to 150 minutes to effectgrowth of a tunnel dielectric 228 having a thickness of from about 1.0nanometers (nm) to about 3.0 nm by oxidation and consumption of theexposed surface of substrate.

In another embodiment a silicon dioxide tunnel dielectric 228 may begrown in a radical oxidation process involving flowing hydrogen (H₂) andoxygen (O₂) gas into a processing chamber at a ratio to one another ofapproximately 1:1 without an ignition event, such as forming of aplasma, which would otherwise typically be used to pyrolyze the H₂ andO₂ to form steam. Instead, the H₂ and O₂ are permitted to react at atemperature approximately in the range of about 900° C. to about 1000°C. at a pressure approximately in the range of about 0.5 to about 5 Torrto form radicals, such as, an OH radical, an HO₂ radical or an Odiradical, at the surface of substrate. The radical oxidation process iscarried out for a duration approximately in the range of about 1 toabout 10 minutes to effect growth of a tunnel dielectric 228 having athickness of from about 1.0 nanometers (nm) to about 4.0 nm by oxidationand consumption of the exposed surface of substrate. It will beunderstood that in this and in subsequent figures the thickness oftunnel dielectric 228 is exaggerated relative to the pad oxide 209,which is approximately 7 times thicker, for the purposes of clarity. Atunnel dielectric 228 grown in a radical oxidation process is bothdenser and is composed of substantially fewer hydrogen atoms/cm³ than atunnel dielectric formed by wet oxidation techniques, even at a reducedthickness. In certain embodiments, the radical oxidation process iscarried out in a batch-processing chamber or furnace capable ofprocessing multiple substrates to provide a high quality tunneldielectric 228 without impacting the throughput (wafers/hr.)requirements that a fabrication facility may require.

In another embodiment, tunnel dielectric layer 228 is deposited bychemical vapor deposition (CVD) or atomic layer deposition and iscomposed of a dielectric layer which may include, but is not limited tosilicon dioxide, silicon oxy-nitride, silicon nitride, aluminum oxide,hafnium oxide, zirconium oxide, hafnium silicate, zirconium silicate,hafnium oxy-nitride, hafnium zirconium oxide and lanthanum oxide. Inanother embodiment, tunnel dielectric 228 is a bi-layer dielectricregion including a bottom layer of a material such as, but not limitedto, silicon dioxide or silicon oxy-nitride and a top layer of a materialwhich may include, but is not limited to silicon nitride, aluminumoxide, hafnium oxide, zirconium oxide, hafnium silicate, zirconiumsilicate, hafnium oxy-nitride, hafnium zirconium oxide and lanthanumoxide.

Referring again to FIG. 2D, a charge-trapping layer is formed on oroverlying the tunnel dielectric 228. Generally, as in the embodimentshown, the charge-trapping layer is a multi-layer charge-trapping layercomprising multiple layers including at least a lower or firstcharge-trapping layer 230 a closer to the tunnel dielectric 228, and anupper or second charge-trapping layer 230 b that is oxygen-lean relativeto the first charge-trapping layer and comprises a majority of a chargetraps distributed in multi-layer charge-trapping layer.

The first charge-trapping layer 230 a of a multi-layer charge-trappinglayer 230 can include a silicon nitride (Si₃N₄), silicon-rich siliconnitride or a silicon oxy-nitride (SiO_(x)N_(y) (HO). For example, thefirst charge-trapping layer 230 a can include a silicon oxynitride layerhaving a thickness of between about 2.0 nm and about 4.0 nm formed by aCVD process using dichlorosilane (DCS)/ammonia (NH₃) and nitrous oxide(N₂O)/NH₃ gas mixtures in ratios and at flow rates tailored to provide asilicon-rich and oxygen-rich oxynitride layer.

The second charge-trapping layer 230 b of the multi-layercharge-trapping layer is then formed over the first charge-trappinglayer 230 a. The second charge-trapping layer 230 b can include asilicon nitride and silicon oxy-nitride layer having a stoichiometriccomposition of oxygen, nitrogen and/or silicon different from that ofthe first charge-trapping layer 230 a. The second charge-trapping layer230 b can include a silicon oxynitride layer having a thickness ofbetween about 2.0 nm and about 5.0 nm, and may be formed or deposited bya CVD process using a process gas including DCS/NH₃ and N₂O/NH₃ gasmixtures in ratios and at flow rates tailored to provide a silicon-rich,oxygen-lean top nitride layer.

As used herein, the terms “oxygen-rich” and “silicon-rich” are relativeto a stoichiometric silicon nitride, or “nitride,” commonly employed inthe art having a composition of (Si₃N₄) and with a refractive index (RI)of approximately 2.0. Thus, “oxygen-rich” silicon oxynitride entails ashift from stoichiometric silicon nitride toward a higher wt. % ofsilicon and oxygen (i.e. reduction of nitrogen). An oxygen rich siliconoxynitride film is therefore more like silicon dioxide and the RI isreduced toward the 1.45 RI of pure silicon dioxide. Similarly, filmsdescribed herein as “silicon-rich” entail a shift from stoichiometricsilicon nitride toward a higher wt. % of silicon with less oxygen thanan “oxygen-rich” film. A silicon-rich silicon oxynitride film istherefore more like silicon and the RI is increased toward the 3.5 RI ofpure silicon.

Referring again to FIG. 2D, the number of dielectric layers furtherincludes a cap layer 232 that is formed on or overlying thecharge-trapping layer 230. In one embodiment, the cap layer 232 includesa silicon nitride all or part of which is subsequently oxidized to forma blocking oxide overlying the charge-trapping layer 230. The cap layer232 can be a single layer of nitride (not shown) having a homogeneouscomposition, a single layer of nitride having a gradient instoichiometric composition, or, as in the embodiment shown, can be amulti-layer cap layer including at least a lower or first cap layer 232a overlying the second charge-trapping layer 230 b, and a second caplayer 232 b overlying the first cap layer 232 a.

In one embodiment, the first cap layer 232 a can include a siliconnitride, a silicon-rich silicon nitride or a silicon-rich siliconoxynitride layer having a thickness of between 2.0 nm and 4.0 nm formedby a CVD process using N₂O/NH₃ and DCS/NH₃ gas mixtures. Similarly, thesecond cap layer 232 b can also include a silicon nitride, asilicon-rich silicon nitride or a silicon-rich silicon oxynitride layerhaving a thickness of between 2.0 nm and 4.0 nm formed by a CVD processusing N₂O/NH₃ and DCS/NH₃ gas mixtures. Optionally, the first cap layer232 a and second cap layer 232 b can comprise different stoichiometries.For example, the second cap layer 232 b can comprise a silicon or oxygenrich composition relative to the first cap layer 232 a to facilitateremoval of the second cap layer in a dry or wet clean process prior tooxidizing the first cap layer. Alternatively, the first cap layer 232 acan comprise a silicon or oxygen rich composition relative to the secondcap layer 232 b to facilitate oxidation of the first cap layer.

Referring to FIG. 2E, a sacrificial oxide layer 234 is formed on oroverlying the second cap layer 232 b. In one embodiment, the sacrificialoxide layer 234 can include a silicon dioxide layer grown by a thermaloxidation process, in-situ steam generation (ISSG), or radicaloxidation, and having a thickness of between 2.0 nm and 4.0 nm. Inanother embodiment, the sacrificial oxide layer 234 can be formed ordeposited by a chemical vapor deposition process in a low pressurechemical vapor deposition (LPCVD) chamber. For example, the sacrificialoxide layer 234 can be deposited by a CVD process using a process gasincluding gas mixtures of silane or DCS and an oxygen containing gas,such as O₂ or N₂O, in ratios and at flow rates tailored to provide asilicon dioxide (SiO₂) sacrificial oxide layer.

Next, referring to FIG. 2F, a patterned mask layer (not shown) is formedon or overlying the sacrificial oxide layer 234, and the sacrificialoxide, cap layer 232, and the charge-trapping layer 230 etched orpatterned to form a gate stack 236 overlying the channel 224 of the NVMtransistor and to remove the sacrificial oxide, cap layer, and thecharge trapping layers 230 from the second region 208 of the substrate204. The patterned mask layer can include a photoresist layer patternedusing standard lithographic techniques, and the sacrificial oxide layer234, cap layer 232, and the charge trapping layers 230 from can beetched or removed using a dry etch process including one or moreseparate steps to stop on the surface 216 of the substrate 204.

Referring to FIG. 1, a gate oxide or GOX preclean is performed, gateoxides for both MOS transistors 214, 212 formed, and a gate layer isdeposited and patterned to form gates for the NVM transistor 226, andboth MOS transistors (step 110). Referring to FIG. 2G, the GOX precleanremoves the sacrificial oxide layer 234 and a portion of the cap layer232 or substantially of all of a top most layer in a multi-layer caplayer are removed from the gate stack 236 in a highly selective cleaningprocess. This cleaning process simultaneously or concurrently furtherremoves any oxide, such as an oxide tunnel dielectric 228 and pad oxide209, remaining in the first region 206 outside the gate stack 236 and inthe second region 208 to prepare the substrate 204 in that region forgate oxide growth. The thickness of the cap layer 232 is adjusted toallow a portion or substantially all of the second cap layer 232 b to beconsumed by the GOX preclean. In one exemplary implementation thesacrificial oxide layer 234 and the second cap layer 232 b are removedin a wet clean process using a 10:1 buffered oxide etch (BOE) containinga surfactant. Alternatively, the wet clean process can be performedusing a 20:1 BOE wet etch, a 50:1 hydrofluoric (HF) wet etch, a padetch, or any other similar hydrofluoric-based wet etching chemistry.

This embodiment of the GOX preclean is advantageous in that itsubstantially does not affect the baseline CMOS process—either in thepreclean step (step 110) or a subsequent oxidation step (step 112), butrather uses it for the integration of the NVM transistor fabrication.

Next, referring to FIG. 2H, an oxidation process is performed to oxidizethe remaining portion of the cap layer 232 or the first cap layer 232 aof a multi-layer cap layer, and a portion of the second charge-trappinglayer 230 b to form a blocking oxide layer 238 overlying the secondcharge-trapping layer. In one embodiment, the oxidation process isadapted to oxidize the first cap layer 232 a to form the blocking oxidelayer 238 while simultaneously or concurrently oxidizing at least aportion of the surface 216 of the substrate 204 in the second region 208to form a first gate oxide 240 overlying at least the channel 218 of atleast one MOS transistor. The oxidation process can includein-situ-steam-generation (ISSG), CVD, or radical oxidation performed ina batch or single substrate processing chamber with or without anignition event such as plasma. For example, in one embodiment theblocking oxide layer 238 and the gate oxide 240 may be grown in aradical oxidation process involving flowing hydrogen (H₂) and oxygen(O₂) gas into a processing chamber at a ratio to one another ofapproximately 1:1 without an ignition event, such as forming of aplasma, which would otherwise typically be used to pyrolyze the H₂ andO₂ to form steam. Instead, the H₂ and O₂ are permitted to react at atemperature approximately in the range of 700-800° C. at a pressureapproximately in the range of 0.5-5 Torr to form radicals, such as, anOH radical, an HO₂ radical or an O diradical radicals at a surface ofthe cap layer 232 or the first cap layer 232 a. The oxidation process iscarried out for a duration approximately in the range of 1-5 minutes fora single substrate using an ISSG process, or 10-15 minutes for a batchfurnace process to effect growth of a blocking oxide layer 238 byoxidation and consumption of the first cap layer 232 a and a portion ofthe second charge-trapping layer 230 b having a thickness of from about3 nm to about 4.5 nm, and gate oxide 240 having a thickness of fromabout 5 nm to about 7 nm.

In some embodiments, such as that shown in FIGS. 2I to 2L, the methodfurther includes a dual gate oxide process flow to enable fabrication ofboth a LV MOS transistor 212 and a HV MOS transistor 214. Referring toFIG. 2I, a patterned mask layer 242 is formed over the first and secondregions 206, 208 of the substrate 204. The patterned mask layer 242 canbe a photoresist layer patterned using standard lithographic techniques,and includes at least one opening 244 over a channel 218 in the secondregion 208. The thick, first gate oxide 240 is etched in the exposedregions by using a BOE etch, under conditions similar to those describedabove with respect to removing the sacrificial oxide layer 234, and thepatterned mask layer 242 is then removed.

Referring to FIG. 2J, the substrate 206 is cleaned using a wet etch thatdoes not etch oxide in order to protect the first gate oxide 240 of theHV MOS transistor 212, and the blocking oxide layer 238 of the gatestack 236. The substrate 206 is then subjected to a thermal oxidationprocess to grow a thin, second gate oxide 246 having a thickness fromabout 1 nm to about 3 nm. In some embodiments, the second gate oxide 246can be overlaid with a deposited layer (not shown) such as siliconoxy-nitride, silicon nitride, aluminum oxide, hafnium oxide, zirconiumoxide, hafnium silicate, zirconium silicate, hafnium oxy-nitride,hafnium zirconium oxide and lanthanum oxide.

Referring to FIG. 2K, a gate layer 248 of any conducting orsemiconducting material suitable for accommodating a biasing of the NVMtransistor 226 and operation of the MOS transistors 214, 212, is formedover the gate stack 236, the first gate oxide 240 of the HV MOStransistor 214, and the second gate oxide 246 of the MOS transistor 212.In one embodiment, the gate layer is formed by physical vapor depositionand is composed of a metal-containing material which may include, but isnot limited to, metal nitrides, metal carbides, metal silicides,hafnium, zirconium, titanium, tantalum, aluminum, ruthenium, palladium,platinum, cobalt and nickel. In another embodiment, the gate layer isformed by a CVD process and is composed of a single doped polysiliconlayer, which may then be patterned to form control gates of the NVMtransistor 226 and MOS transistors 214, 212.

Referring to FIG. 2L, the gate layer 248 is patterned using a mask layer(not shown) and standard lithographic techniques to stop on surfaces ofthe blocking oxide layer 238, the first gate oxide 240 and the secondgate oxide 246, thereby forming a gate 250 for the gate stack 236 of aNVM transistor 226, a gate 252 for the HV MOS transistor 214, and a gate254 for the MOS transistor 212.

Referring to FIG. 1 and FIG. 2M, a first spacer layer is deposited andetched to form first sidewall spacers 256 adjacent to the gates 252,254, of the MOS transistors 212, 214, and the NVM transistor 226, andone or more lightly-doped drain extensions (LDD 258) are implantedadjacent to and extend under sidewall spacers 256 of one or more of theMOS transistors 212, 214 (step 112).

Next, a SONOS LDD mask is formed over the substrate 204 andlightly-doped drain extensions (LDD 260) are implanted, adjacent to theNVM transistor 226. Finally, a second spacer layer is deposited andetched to form second sidewall spacers 262 adjacent to the gate stack236, of the NVM transistor 226 (step 114).

Referring to FIGS. 1 and 2N, with the NVM transistor 226, HV MOStransistor 214 and MOS transistor 212 substantially complete, source anddrain implants are performed to form source and drain regions 264 forall transistors and a silicide process performed (step 116). Asdepicted, silicide regions 266 may be formed on the exposed gates 250,252 and 254 and exposed source and drain regions 264. The silicideprocess may be any commonly employed in the art, typically including apre-clean etch, cobalt or nickel metal deposition, anneal and wet strip.

Referring FIG. 1 and FIG. 2N, optionally the method of fabricatingmemory cells including an embedded or integrally formed SONOS based NVMtransistor and MOS transistors further includes the step of forming astress inducing layer or structure 268, such as a stress inducingnitride layer, over the gate stack 236 of the NVM transistor 226 toincrease data retention and/or to improve programming time andefficiency (step 118). In particular, inducing stress into thecharge-trapping layer 230 of the NVM transistor 226 changes energylevels of charge traps formed therein, thereby increasing chargeretention of the charge-trapping layer. In addition, forming a stressinducing structure 268, in or on the surface 216 of the substrate 204proximal to, and preferably surrounding, a region of the substrate inwhich the channel 224 of NVM transistor 226 is formed will reduce theband gap, and, depending on the type of strain, increases carriermobility. For example, tensile strain, in which inter-atomic distancesin the crystal lattice of the substrate 204 are stretched, increases themobility of electrons, making N-type transistors faster. Compressivestrain, in which those distances are shortened, produces a similareffect in P-type transistors by increasing the mobility of holes. Bothof these strain induced factors, i.e., reduced band gap and increasedcarrier mobility, will result in faster and more efficient programmingof NVM transistor 226.

The strain inducing structure 268 can include a pre-metal dielectric(PMD) layer formed using a High Aspect Ratio Process (HARP™) oxidationprocess, a compressive or tensile nitride layer formed using a plasmaenhanced chemical vapor deposition (PECVD) or a Bis-TertiaryButylAminoSilane (BTBAS) nitride layer.

In certain embodiments, such as that shown in FIG. 2N, the stressinducing structure 268 may also be formed over one or more of the MOStransistors to induce strain in the channel of the MOS transistor.

Finally, the standard or baseline CMOS process flow is continued tosubstantially complete the front end device fabrication (step 120),yielding the structure shown in FIG. 2N. FIG. 2N is a block diagramillustrating a cross-sectional view of a portion of a finished memorycell including an embedded SONOS based NVM transistor and MOStransistors fabricated according to the method of FIGS. 1 and 2A-2M.

FIGS. 3A and 3B are graphs illustrating improvements in thresholdvoltage (V_(T)) uniformity of an NVM transistor including an indiumchannel according to an embodiment of the present disclosure. ReferringFIGS. 3A and 3B it is noted that the heavier Indium atom does not moveor diffuse as much as a light Boron atoms used in prior n-channel SONOSbased NVM transistors, in the subsequent high temperature steps andhence the random dopant effects are reduced, significantly improving theV_(T) uniformity from a sigma of V_(T) from ˜150 mV to ˜70 to 80 mV.

FIGS. 4A-4C are graphs illustrating basic program erase characteristicsand showing improvements in endurance and data retention for a memorydevice including an embedded SONOS based NVM transistor fabricatedaccording to an embodiment of the present disclosure. In particular,FIG. 4A shows subthreshold characteristics of the SONOS based NVMtransistor in both the programmed and erased states. This figure shows agood subthreshold slope of about 135 mV is achieved using Indium implantwhich is one of the embodiments of the present disclosure.

FIG. 4B shows the cycling endurance of the SONOS NVM transistorfabricated according to an embodiment of the process describedpreviously. It can be seen from FIG. 4B that there is no significantchange in threshold voltage (V_(T)) on either program or erase after 100thousand program and erase (P/E) cycles.

FIG. 4C shows the data retention charge loss for SONOS based NVMtransistor fabricated according to an embodiment of the presentdisclosure. Referring to FIG. 4C, the V_(T) window, i.e., a differencebetween the threshold voltage in programmed and erased states, is seento be greater than 1V, which gives sufficient margin for error-free readeven after 10 years or 3E8 seconds.

Thus, embodiments of memory cells including embedded or integrallyformed SONOS based NVM transistor and MOS transistors and methods offabricating the same have been described. Although the presentdisclosure has been described with reference to specific exemplaryembodiments, it will be evident that various modifications and changesmay be made to these embodiments without departing from the broaderspirit and scope of the disclosure. Accordingly, the specification anddrawings are to be regarded in an illustrative rather than a restrictivesense.

The Abstract of the Disclosure is provided to comply with 37 C.F.R.§1.72(b), requiring an abstract that will allow the reader to quicklyascertain the nature of one or more embodiments of the technicaldisclosure. It is submitted with the understanding that it will not beused to interpret or limit the scope or meaning of the claims. Inaddition, in the foregoing Detailed Description, it can be seen thatvarious features are grouped together in a single embodiment for thepurpose of streamlining the disclosure. This method of disclosure is notto be interpreted as reflecting an intention that the claimedembodiments require more features than are expressly recited in eachclaim. Rather, as the following claims reflect, inventive subject matterlies in less than all features of a single disclosed embodiment. Thus,the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment.

Reference in the description to one embodiment or an embodiment meansthat a particular feature, structure, or characteristic described inconnection with the embodiment is included in at least one embodiment ofthe circuit or method. The appearances of the phrase one embodiment invarious places in the specification do not necessarily all refer to thesame embodiment.

What is claimed is:
 1. A method comprising: forming a dielectric stackon a substrate, the dielectric stack including a tunneling dielectric onthe substrate, a charge-trapping layer on the tunneling dielectric, acap layer on the charge-trapping layer and a sacrificial oxide on thecap layer; patterning the dielectric stack to form a gate stack of anon-volatile memory (NVM) transistor of a memory device in a firstregion of the substrate while concurrently removing the dielectric stackfrom a second region of the substrate; performing a gate oxidation (GOX)preclean to remove the sacrificial oxide and at least a top portion ofthe cap layer from the gate stack of the NVM transistor whileconcurrently removing a pad oxide from the second region of thesubstrate, wherein the top portion of the cap layer comprises an oxygenrich composition relative to a bottom portion of the cap layer tofacilitate removal of the top portion of the cap layer; and performing agate oxidation process to form a gate oxide of ametal-oxide-semiconductor (MOS) transistor overlying the substrate inthe second region while concurrently oxidizing the bottom portion of thecap layer to form a blocking oxide overlying the charge-trapping layer.2. The method of claim 1 further comprising forming a source and drainadjacent to the gate stack of the NVM transistor, wherein forming thesource and drain comprises performing one or more of a lightly-dopeddrain (LDD) implants.
 3. The method of claim 2 further comprisingforming from a single patterned polysilicon layer a control gate for theNVM transistor and a gate for the MOS transistor.
 4. The method of claim1 further comprising prior to forming the dielectric stack on thesubstrate, implanting Indium in the first region of the substrate toform a channel of the NVM transistor.
 5. The method of claim 4 whereinimplanting Indium to form the channel of the NVM transistor comprisesimplanting Indium to a concentration selected to improve a thresholdvoltage (V_(T)) uniformity of the NVM transistor from a sigma of V_(T)from about 150 millivolts (mV) to from about 70 to 80 mV.
 6. The methodof claim 1 further comprising forming a stress inducing nitride layerover the gate stack of the NVM transistor to change energy levels ofcharge traps formed in the charge-trapping layer to increase chargeretention of the charge-trapping layer.
 7. The method of claim 6 whereinforming the stress inducing nitride layer over the gate stack of the NVMtransistor further comprises forming the stress inducing nitride layerover the MOS transistor to induce stress in a channel of the MOStransistor.
 8. The method of claim 1 wherein the charge-trapping layercomprises multiple layers including a lower charge-trapping layer closerto the tunneling dielectric comprising a nitride, and an uppercharge-trapping layer overlying the lower charge-trapping layer thatcomprises a nitride and is oxygen-lean relative to the lowercharge-trapping layer.
 9. The method of claim 1, wherein the cap layercomprises a single layer of nitride having a gradient in stoichiometriccomposition to form the top portion of the cap layer and the bottomportion of the cap layer.
 10. A method comprising: implanting Indium ina first region of a substrate to form a channel of a non-volatile memory(NVM) transistor of a memory device; forming a dielectric stack on thesubstrate overlying the channel, the dielectric stack including atunneling dielectric on the substrate and a charge-trapping layer on thetunneling dielectric, a multi-layer cap layer including a first caplayer comprising silicon-nitride overlying the charge-trapping layer,and a second cap layer comprising silicon-nitride overlying the firstcap layer, wherein the second cap layer is oxygen rich relative to thefirst cap layer; and patterning the dielectric stack to form a gatestack of the NVM transistor in the first region of the substrate and toremove the dielectric stack from a second region of the substrate. 11.The method of claim 10 wherein implanting Indium to form the channel ofthe NVM transistor comprises implanting Indium to a concentrationselected to improve a threshold voltage (V_(T)) uniformity of the NVMtransistor from a sigma of V_(T) from about 150 millivolts (mV) to fromabout 70 to 80 mV.
 12. The method of claim 10 wherein thecharge-trapping layer comprises multiple layers including a lowercharge-trapping layer closer to the tunneling dielectric comprising anitride, and an upper charge-trapping layer overlying the lowercharge-trapping layer that comprises a nitride and is oxygen-leanrelative to the lower charge-trapping layer.
 13. The method of claim 10,further comprising performing a gate oxidation (GOX) preclean to removethe second cap layer from the gate stack of the NVM transistor whileconcurrently removing a pad oxide from the second region of thesubstrate, wherein the oxygen rich composition of the second cap layerfacilitates removal of the second cap layer.
 14. The method of claim 13further comprising performing an oxidation process to form a gate oxideof a metal-oxide-semiconductor (MOS) transistor overlying the substratein the second region while concurrently forming a blocking oxideoverlying the charge-trapping layer, wherein forming the blocking oxidecomprises oxidizing the first cap layer.
 15. The method of claim 14further comprising forming a stress inducing nitride layer over the gatestack of the NVM transistor to change energy levels of charge trapsformed in the charge-trapping layer to increase charge retention of thecharge-trapping layer.
 16. The method of claim 15 wherein forming astress inducing nitride layer over the gate stack of the NVM transistorfurther comprises forming the stress inducing nitride layer over the MOStransistor to induce stress in a channel of the MOS transistor.
 17. Themethod of claim 14 further comprising forming from a single patternedpolysilicon layer a control gate for the NVM transistor and a gate forthe MOS transistor.
 18. A method comprising: forming a dielectric stackon a substrate, the dielectric stack including a tunneling dielectric onthe substrate, a charge-trapping layer on the tunneling dielectric, amulti-layer, silicon-nitride cap layer including a first cap layeroverlying the charge-trapping layer, and a second cap layer overlyingthe first cap layer, and a sacrificial oxide on the second cap layer;patterning the dielectric stack to form a gate stack of a non-volatilememory (NVM) transistor of a memory device in a first region of thesubstrate while concurrently removing the dielectric stack from a secondregion of the substrate; and performing a gate oxidation (GOX) precleanto remove the sacrificial oxide and second cap layer from the gate stackof the NVM transistor while concurrently removing a pad oxide from thesecond region of the substrate, wherein the second cap layer is oxygenrich relative to the first cap layer to facilitate removal of the secondcap layer.
 19. The method of claim 18, further comprising performing agate oxidation process to form a gate oxide of ametal-oxide-semiconductor (MOS) transistor overlying the substrate inthe second region while concurrently forming a blocking oxide overlyingthe charge-trapping layer, wherein forming the blocking oxide comprisesoxidizing the first cap layer.
 20. The method of claim 18, wherein thecharge-trapping layer comprises a multi-layer charge-trapping layerincluding a on the tunneling dielectric, a first charge-trapping layerformed on the tunneling dielectric, and a second charge-trapping layerthat is oxygen-lean relative to the first charge-trapping layer andcomprises a majority of a charge traps distributed in multi-layercharge-trapping layer formed on the first charge-trapping layer.